This invention relates to a connectionless communication network and in particular to an arrangement and method for providing communication in a network structure comprising a ground network and satellite links.
Satellite communications systems employing geostationary satellites carrying traffic between ground stations are well established. Although such systems are effective in operation, they suffer from two limitations. Firstly, a geostationary satellite occupies an equatorial orbit and thus its xe2x80x98footprintxe2x80x99 may not extend to higher latitudes. Secondly, a large number of geostationary satellites have already been installed and there are now very few orbital positions available for new satellites. To address these problems, there are recent proposals to provide satellite systems in which non-geostationary satellite nodes co-operate with a ground-based network to form a global communications system.
A method of providing communications in a system employing non-geostationary satellites is described in specification No U.S. Pat. No. 5,621,415. In that arrangement, the satellite footprints are partitioned into linear spanning cells containing multiple linear segments to reduce the hand-off overhead.
A method of operating an ATM network by using a software representation comprising a virtual network model is described in specification No. EP-A-748142. A description of ATM-based routing in LEO/MEO satellite networks is given by M Werner et al. in IEEE Journal on Selected areas in Communications, Vol. 15, No. 1, January 1997, pages 69-82.
Our co-pending United Kingdom patent application No 9707832.3 (Rosenberg 1) relates to a connectionless communications network for transporting broad band services requiring quality of service (QoS) guarantees and for ensuring that the service maintains the quality of service determined at the start of a communication session throughout the duration of that session. In that application, moving satellite nodes are accommodated by maintaining a virtual model of the network having fixed virtual nodes to control packet routing. Routing of packets is performed with reference to the virtual network. By ensuring that topology changes re-establish the bandwidth available within the virtual model, the system provides a guaranteed quality of service. The present invention relates to the protocols for such a network and discloses a method that allows B-ISDN signalling to be used.
Connectionless network operation has been traditionally used in data communications. Its main features are that there is no requirement for a connection set-up phase and that each packet of a connection is routed independently. No connection related state is then needed in the switch and a topological change in a node (i.e., its failure or its hand-over) only necessitates the update of the routing tables contained in its neighbours. For example, networks based on the Internet Protocol (IP) are typically connectionless. Wireless and satellite data networks are often designed for connectionless operation in order to avoid the need for explicit connection hand-overs due to the physical mobility of users or systems. Hence, in a satellite network, connectionless operation removes the need for connection related states to be held in the sky segment. Each packet of information is self-routed so the network is able to adapt to changes due to hand-overs between satellites. Each hand-over can be interpreted as a change of topology.
Transport of broad band services requiring quality of service (QoS) guarantees (i.e., multimedia services) over connectionless network, such as the Internet, is a major issue since traditionally this kind of network has only offered best-effort services. QoS guarantees are usually provided through connection orientation, as in the case of BISDN networks based on ATM. For such networks, a virtual circuit is established during a set-up phase and then routing of individual packets is simplified (i.e., no decision needs to be made) since the route (i.e., the virtual circuit) has already been chosen. In that case, the trade-off between connection orientation and connectionless operation is: the need for a call set-up phase; much less processing per packet; the necessity to keep connection related state in the switches involved in the connection; easier handling of QoS guarantees; no need for end to end re-sequencing; and the need to perform a new connection set-up phase for any topological change involving a node in the route. This last feature is crucial for a LEO based satellite network where topological changes are very frequent.
An object of the present invention is to provide a connectionless digital communication network having changeable topology and means for maintaining the bandwidth available when the topology changes.
According to one aspect of the invention there is provided a method of operating a non-geostationary satellite network incorporating a plurality of satellite nodes and ground stations in communication with said nodes, the method comprising providing a virtual model of the network, and determining within said virtual model a capacity of space segment band width for different traffic QoS classes, and determining the relationship between the virtual model and the network so as to transfer the capacity determination to the network.
QoS guarantees can be achieved without requiring a connection oriented function within the network so that the network endpoints remain insensitive to topological change within the network.
In a preferred embodiment, a fixed virtual model of the network is maintained and in a physical nodes advertises to adjacent physical nodes with which it has direct links, which virtual node it embodies.
The network preferably includes topology groups of virtual nodes, being fully interconnected within the topology group within the model. Services are preferably ATM based.
In one implementation, the network nodes include satellites with onboard switching functions.
In another implementation the network is ground based.
A connection control system preferably directs connectionless packets to follow paths selected from a virtual model of the network.
In one preferred alternative, a user network interface is part of the Plesiochronous Digital Hierarchy (PDH). In another alternative, a user network interface is part of the Synchronous Digital Hierarchy (SDH or SONET). In yet another alternative, a user network interface is based on the Asynchronous Transmission Mode.
The invention extends to a connection control system for a connectionless communication network, said control system acting to direct connectionless packets to follow paths selected from a virtual model of the network.
The invention further provides a method for transporting broad band services requiring Quality of Service (QoS) guarantees on a connectionless network and ensuring that the service maintains the Quality of Service determined at the start of a communication session throughout the duration of that session.
The invention further provides a call admission control (CAC) policy that ensures a grade of service in admitting calls and in the maintenance of calls.
In a preferred embodiment of the invention, the physical network consists of moving nodes. A virtual model of the network, consisting of fixed Virtual Nodes, is maintained in a connection control system. This model is used to control the route that packets will take through the network. As topology changes happen in the network then the Virtual Nodes of the model become embodied by different real nodes and communicate their virtual identity to connected nodes. Routing of the packets is performed with reference to the virtual network so that packet routing follows the current embodiment of the virtual network. By ensuring that the topology changes re-establish the bandwidth available within the virtual model then the system guarantees that the quality of service established for the session is maintained.